Coral bleaching continues to plague Australia's Great Barrier Reef

After intensive aerial and underwater surveys, about 35 percent of the corals in the northern and central Great Barrier Reef are found to have perished due to mass bleaching, according to scientists.

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The decline is currently on-going, and varies from north to south along the reef's 2,300km length.

“We found on average, that 35 percent of the corals are now dead or dying on 84 reefs that we surveyed along the northern and central sections of the Great Barrier Reef, between Townsville and Papua New Guinea. Some reefs are in much better shape, especially from Cairns southwards, where the average mortality is estimated at only 5 percent,” said Professor Terry Hughes, Director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University (JCU).

“This year is the third time in 18 years that the Great Barrier Reef has experienced mass bleaching due to global warming, and the current event is much more extreme than we've measured before. These three events have all occurred while global temperatures have risen by just 1 degree Celsius above the pre-industrial period. We're rapidly running out of time to reduce greenhouse gas emissions,” he continued.

However, if ocean temperatures drop, the zooxanthellae would be able to re-colonise the bleached corals, which then regenerates.

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“Fortunately, on reefs south of Cairns, our underwater surveys are also revealing that more than 95 percent of the corals have survived, and we expect these more mildly bleached corals to regain their normal colour over the next few months,” said Dr Mia Hoogenboom, also from JCU.

“In Western Australia, bleaching and mortality is also extensive and patchy. On the Kimberly coast where I work, up to 80 percent of the corals are severely bleached, and at least 15 percent have died already,” said Dr Verena Schoepf from The University of Western Australia.

The researchers will revisit the same reefs over the coming months to measure the final loss of corals due to the bleaching. They expect the corals to recover in a decade or so, but the recovery period will be longer for the oldest and largest corals.